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my @match = do {
    open my $fh, '<', $file or die "$!";
    grep /^$ARGV[0]\|/, <$fh>;
};

my @match;
{
...
    my $rx = qr/^$ARGV[0]\|/
    m/$rx/ && push @match, $_ while <$fh>;
}

sub grep_file {
    my ($rx, $file) = @_;
...
};

my @match = grep_file($ARGV[0], $file);