Here's one way of doing it (Note the update at the end): 

#!/usr/bin/env perl

use strict;
use warnings;

my $text;
my $match = 0;
my $regex = qr/int\s+main\s*\(/;

{
    local $/;                       # enable "slurp" mode
    $text = <DATA>;                  # read whole file
}

while ($text =~ /$regex/gms)
{
    print "Match found on line ",
            scalar ( split /\n/, substr $text, 0, pos($text) )
                - scalar ( split /\n/, ${^MATCH} ) + 1,
            "\n";
    $match = 1;
}

print "No matches found\n" unless $match;

__DATA__
/* Dummy multi-main C code */

int
main(int argc, char** argv)

int main(int argc, char** argv)

/* END */

[download]

Output:

$ pm_multi_line_match.pl
Match found on line 3
Match found on line 6

[download]

Update: As the arithmetic operators already provide a scalar context, the two calls to scalar() are redundant; so you could just write: 

    print "Match found on line ",
            split(/\n/, substr $text, 0, pos $text) - split(/\n/, ${^M
+ATCH}) + 1,
            "\n";

[download]

-- Ken

In reply to Re: Getting the line numbers of a multi-line match by kcott
in thread Getting the line numbers of a multi-line match by Athanasius

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