foreach (@line_in body) { # quasi-pseudocode
 if (|(<!--#include virtual="([^"]+)"-->|) {
   #$1 contains the whole match, 
   # $2 what's in between the quotes
   # so delete $1 unless $2 matches the pattern
   s/$1// unless $2 =~ m|/global_nav\w*\.ssi|;
 }
}