v3.25.3
Cover - $ / shares
7 Months Ended
Sep. 30, 2025
Nov. 14, 2025
Nov. 12, 2025
Document Type 10-Q    
Amendment Flag false    
Document Quarterly Report true    
Document Transition Report false    
Document Period End Date Sep. 30, 2025    
Document Fiscal Period Focus Q3    
Document Fiscal Year Focus 2025    
Current Fiscal Year End Date --12-31    
Entity File Number 001-42642    
Entity Registrant Name Perimeter Acquisition Corp. I    
Entity Central Index Key 0002061473    
Entity Tax Identification Number 98-1843886    
Entity Incorporation, State or Country Code E9    
Entity Address, Address Line One 4514 Cole Avenue    
Entity Address, Address Line Two Suite 1600    
Entity Address, City or Town Dallas    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 75204    
City Area Code (512)    
Local Phone Number 200-2533    
Entity Current Reporting Status No    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company true    
Elected Not To Use the Extended Transition Period false    
Entity Shell Company true    
Inception date Mar. 06, 2025    
Units, each consisting of one Class A ordinary share, $0.0001 par value, and one-half of one redeemable warrant [Member]      
Title of 12(b) Security Units, each consisting of one Class A ordinary share, $0.0001 par value, and one-half of one redeemable warrant    
Trading Symbol PMTRU    
Security Exchange Name NASDAQ    
Class A ordinary shares included as part of the units [Member]      
Title of 12(b) Security Class A ordinary shares included as part of the units    
Trading Symbol PMTR    
Security Exchange Name NASDAQ    
Redeemable warrants included as part of the units, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50 [Member]      
Title of 12(b) Security Redeemable warrants included as part of the units, each whole warrant exercisable for one Class A ordinary share at an exercise price of $11.50    
Trading Symbol PMTRW    
Security Exchange Name NASDAQ    
Common Class A [Member]      
Entity Common Stock, Shares Outstanding     24,788,000
Entity Listing, Par Value Per Share $ 0.0001    
Common Class B [Member]      
Entity Common Stock, Shares Outstanding   6,037,500  
Entity Listing, Par Value Per Share $ 0.0001