These are modifiers allow you to tell Perl how to interpret the characters in a match.
# /a - use the ASCII interpretation
# /u - use the Unicode interpretation
# /l - respect the locale
# A single /a modifier affects the character class shortcuts,
# but having two /a modifiers also tells Perl to use ACII-only case-folding.
\!h # Some examples:
/k/aai # only matches the ASCII K or k, not Kelvin sign
/k/aia # the /a's don't need to be next to each other
/ss/aai # only matches ASCII ss, SS, sS, Ss, not β
/ff/aai # only matches ASCII ff, FF, fF, Ff, not ff