You could use print instead of system and echo:

print "\e[36;38mWELCOME\e[37m TO PERLMONKS\n";
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And - if you need it more than once - you might want to put the respective ANSI escape sequences in variables, so it's easier to type and read:

my $UL = "\e[36;38m";
my $UE = "\e[37m";

print "${UL}WELCOME$UE TO PERLMONKS\n";
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