use strict;
use warnings;

print "\n";

scalar @ARGV == 2
    or die "USAGE: perl $0 <filename> <regex>\n";

open(my $fh, '<', $ARGV[0])
    or die "Unable to open file '$ARGV[0]' for reading: $!";

my $match = 0;
my $regex = qr/$ARGV[1]/;
my @lines = <$fh>;                  # read whole file

foreach (0 .. $#lines)
{
    if ($lines[$_] =~ /$regex/)
    {
        printf "Match found on line %d\n", ($_ + 1);
        $match = 1;
    }
}

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

##</code><code>##

>perl findinfile.pl run.c "int\s+main\s*\("

##</code><code>##

int
main(int argc, char** argv)

##</code><code>##

...
my $text;
my $match = 0;
my $regex = qr/$ARGV[1]/;

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

while ($text =~ /$regex/gms)
{
    print "Match found\n";
    $match = 1;
}

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