TIMTOWTDI. It's pretty simple so long as you assign to a lexical in your while loop. eg:

#!/usr/bin/env perl 
use strict;
use warnings;

my @substrings = qw/XLON_DBX1AE_USD XLON_DBX1AE_GBP/;

while (my $line = <DATA>) {
    if (grep { index ($line, $_) > -1 } @substrings) {
        print "Found a match in line: $line";
    }
}

__DATA__
strategies{
XLON_DBX0F2_GBP DynamicSpreadQuoter {
        tradingServiceAttributes { session LSE1 }
        fairPrice {
        securityID LU0490618542
        securityIDSource 4
        service IDNPS
        }
}
XLON_DBX1AE_GBP DynamicSpreadQuoter {
        tradingServiceAttributes { session LSE3 }
        fairPrice {
        securityID LU0322252127
        securityIDSource 4
        service NMP_ABPS
        }
}
XLON_DBX1AE_USD DynamicSpreadQuoter {
        tradingServiceAttributes { session LSE1 }
        fairPrice {
        securityID LU0322252171
        securityIDSource 4
        service NMP_ABPS
        }
}
;;
}
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I've assumed here that your patterns are all substrings rather than regular expressions.