You can interpolate precompiled regexes into substitutions as well. The different modifiers can go into each regex: 
my @SUB_REC = (
   qr{(?:this|or|that)}si,
   qr{(?:One|Two|Three)}s,
);
my @SUBSTITUTION = qw/bob four/;
for my $idx (0..$#SUB_REC) {
    $str =~ s/$SUB_REC[$idx]/$SUBSTITUTION[$idx]/;
}

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In reply to Re: Precompiling substitution regex by moritz
in thread Precompiling substitution regex by FreakyGreenLeaky

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