print $_ if( $_ =~ m/$keyword/ );

##</code><code>##

my $pattern = join( '|', @keywords );
if( $_ =~ m/$pattern/ )

##</code><code>##

use strict;
use warnings;

my @keywords = qw( keyword1 keyword2 keyword3 );
my $pattern = join( '|', map quotemeta, @keywords );
print "pattern = [$pattern]\n";

# using 'if' as a modifier
while( my $line = <DATA> )
{
    print $line if ( $line =~ m/$pattern/ );
}

# using an 'if' block
while( my $line = <DATA> )
{
    if( $line =~ m/$pattern/ )
    {
        print $line;
    }
}

__DATA__
somelines...
somelines...
somewords...keyword1..somewords
somelines...
somewords...
keyword2...somewords...

##</code><code>##

pattern = [keyword1|keyword2|keyword3]
somewords...keyword1..somewords
keyword2...somewords...