v3.26.1
Cover - $ / shares
 6 Months Ended
Jun. 30, 2026
Aug. 14, 2026
Document Type 10-Q  
Amendment Flag false  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Jun. 30, 2026  
Document Fiscal Period Focus Q2  
Document Fiscal Year Focus 2026  
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 12700 Hillcrest Rd  
Entity Address, Address Line Two Suite 125  
Entity Address, City or Town Dallas  
Entity Address, State or Province TX  
Entity Address, Postal Zip Code 75230  
City Area Code (512)  
Local Phone Number 200-2533  
Entity Current Reporting Status Yes  
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  
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