package GreppedFileHandle;
use strict;

sub TIEHANDLE {
	local *FH;
	open my $self, \*FH;
    bless $self, shift
}

sub PRINT {
    my $self = shift;

    my $output = "@_";

    if( $output =~ /\Q$::GREP_STR\E/ ) {
        print( STDOUT $output );
    } else {
        print "Swallowed a line\n";
    };
}

package main;
our $GREP_STR = '123'; # needs to be 'our', not 'my'

tie *grep_fh, 'GreppedFileHandle';

print STDOUT "Header - always print this\n";
print grep_fh "My line with 123\n";    ## printed at the terminal
print grep_fh "My line with 456\n";    ## not printed