To give you exact code, I'd want to see your data. That often gives clues as to the best way to approach the problem. But here's a general solution:

use strict;
use warnings;

{
local $/ = undef;    ### Set line terminator to nothing
open(HANDLE, 'test.txt') || die;
$_ = <HANDLE>;       ### Slurp entire file
}

### Match everything at or following first instance of pattern
if (m/(my pattern.*)/s) {
    print $1;
}
else {
    print "No match found.\n";
}
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