Are you looking for part of a string or the whole string? If you are looking for the whole string it is better to use eq because a regular expression is slower.

if ( $a eq 'abcdefgh' or $b eq 'abcdefgh' or $c eq 'abcdefgh') {

}
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Also shorter code does not necessary mean it is easier to read. Taking your code and putting it into a tabular format,

if ( $a =~ /abcdefgh/ ||
     $b =~ /abcdefgh/ ||
     $c =~ /abcdefgh/ ||
     ....... ) {

}
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One more thing, if you are trying to match the same pattern, put the pattern in a string and match against it. Name the pattern something that will make sense for the program can also enhance readability.

my $USER_NAME = qr{abcdefgh}xms;

if ($a =~ $USER_NAME ||
    $b =~ $USER_NAME ||
    $c =~ $USER_NAME) {

}
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