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";
}